Corrosion resistant metallic articles

ABSTRACT

INCLUSION OF ELEMENTAL ANODE MATERIALS INTO CATHODIC METALLIC MATRICES WILL GIVE SIGNIFICANT IMPROVEMENTS IN CORROSION RESISTANCE, PROVIDED THAT THE ELEMENTAL ANODE WILL CORRODE ANODICALLY, YIELD A COLLOIDAL OR FINELY DISPERSED HYDROUS METAL OXIDE IN THE CORROSIVE ELECTROLYTE WHICH WILL ASSUME A POSITIVE CHARGE. SUCH POSITIVELY CHARGED HYDROUS   OXIDES WILL MIGRATE ELECTROPHORETICALLY AND DEPOSIT PROTECTIVE FILMS ON THE MATRIX METAL CATHODE.

April 6, 1971 J, PRYQR 3,574,081

CORROSION RESISTANT METALLIC ARTICLES Filed Jan. 12, 1968 [Z 1 M l4 /7 M /2 pl/ J2 INVENTOR 2 MICHAEL J. PPYOP ATTORNEY 3,574,081 CORROSION RESISTANT METALLIC ARTICLES Michael .I. Pryor, Woodbridge, Conn, assiguor to Olin Corporation Filed Jan. 12, 1968, Ser. No. 697,479 Int. Cl. C23f 13/00 US. Cl. 204--197 9 Claims ABSTRACT OF THE DISCLOSURE Inclusion of elemental anode materials into cathodic metallic matrices will give significant improvements in corrosion resistance, provided that the elemental anode will corrode anodically, yield a colloidal or finely dispersed hydrous metal oxide in the corrosive electrolyte which will assume a positive charge. Such positively charged hydrous oxides will migrate electrophoretically and deposit protective films on the matrix metal cathode.

Many pure metals such as iron and copper show relatively low resistance to corrosion in severe electrolytes. Classically, three basic mechanisms have been used in alloy development to improve the corrosion resistance of such metals.

The most widely used means of allowing to improve corrosion resistance involves the addition of solid solution elements which have a high afiinity for oxygen. In the case of copper, addition of such elements as nickel and aluminum, with or without zinc, result in the formation of quite protective oxide films upon the copper alloy. In the case ofiron the protective film forming additions are chromium, with or without nickel, which similarly forms highly protective films primarily composed of chromic oxides. Such alloys possessing high intrinsic corrosion resistance due to the presence of a protective oxide film, will, characteristically, show low total weight losses in corrosive environments, together with excellent resistance to concentration cell corrosion. They will, however, invariably, and under the appropriate conditions, be subject to quite intense local attack, maifesting itself as pitting, or, in the presence of solid obstructions, as crevice corrosion.

In an effort to alleviate sensitivity to localized corrosion, which is generally observed in mildly reducing corrosive media, elemental additions of cathodic noble metals have been added in small concentrations of the order of about /2% by weight. Accordingly, additions of platinum and palladium have proved quite effective in improving the pitting corrosion characteristics of stainless steels and titanium in reducing environments. This work has been reported in detail by M. Stern and H. Wissenberg, Journal of the Elecrochemical Society, 106, 755 and 759 (1959) and N. D. Greene, C. R. Bishop and M. Stern, Journal of the Electrochemical Society, 108, 836 (1961).

It is also known that the corrosion rates of metals such as copper and iron can be controlled in corrosive environments by coupling to sacrificial anodes. Here, typical anodes would include zinc, magnesium and aluminum. The function of the sacrificial anode is to lower the potential of the metal to be protected into a range where corrosion is thermodynamically impossible.

I have discovered a fourth and totally novel method of providing protection to a wide variety of metals whose corrosion resistance in a particular environment is relatively low. Basically, the method comprises providing at least one of a series of elemental additions to the metal to be protected. These elemental additions are selected from a group of pure metals or alloys whose corrosion potential in the particular environment is at least 10 millivolts more active than that of the matrix metal. The quantity of elemental anode material to be provided should be between .005 and 25% by weight. More optimally, the

United States Patent ice amount of second phase elemental anode should lie between 0.1 and 5% by weight. The resulting product is a mixture of the matrix metal or alloy interspersed with anode material. There are further restrictions upon the elemental anode other than that it is anodic to the matrix metal.

When the elemental anode corrodes galvanically and passes its ions into solution, these metal ions must be capable of interacting with the electrolyte by a hydrolysis mechanism to provide a stable and insoluble hydrous oxide. Furthermore, the hydrous oxide must initially be formed either in colloidal or very finely dispersed form and must be able to pick up a positive charge from solution. The positively charged colloid or fine precipitation will then migrate electrophoretically as a large cation of low transference number. The flow of corrosion current on the corroding specimen will drive the positively charged particles to the cathodic matrix material upon which deposition of a film of hydrous oxide will be obtained. The hydrous oxide may, however, contain anions from the particular electrolyte in which it is formed.

Such deposited films of hydrous oxide will provide substantial protection against pitting, crevice corrosion, erosion-corrosion, stress corrosion, and intergranular corrosion.

It is to be emphasized that the formation of such deposited hydrous oxide films by electrophoresis is a self-limiting process. A certain amount of initial corrosion is required to form the film. Such hydrous oxide films appear to exhibit little or no resistance to the outward migration of cations but exhibit high resistance to the passage of electrons and, therefore, polarize the cathodic portion of the overall corrosion reaction most strongly. This has the advantage that once such a film is ruptured locally the remaining specimen does not become an effective cathode thereby leading to pitting. Accordingly, corrosion is limited to the area of the specimen which has been exposed to action of the electrolyte and a new and similar film reforms rapidly. This is the basic reason why such films are most advantageous in protecting against pitting.

Thus, as shown schematically in the drawing, a matrix 10 is provided having interspersed therein a plurality of elemental anode particles 11. At certain points indicated at 13a and 13b on the surface 12 of the matrix, anode particles 11 will be exposed to electrolyte 14.

Thus, in the electrolyte, the anodic members at 13a and 1312 will react with the electrolyte according to the reaction The metal hydroxide formed is highly dispersed and is insoluble. Furthermore, the metal hydroxide, shown in exaggerated form at 15, picks up a positive charge from the solution.

The positively charged, dispersed metal hydroxide will migrate to the cathodic matrix 16 and will deposit thereon. Thus, a hydroxide film 17 is formed. The deposition just described will continue until the anodic particles 11 at the surfae 12 are exhausted or until the electronic resistance of the hydroxide film is high enough to stifle further corrosion.

It might appear that a corrosive cell could be set up between the potentially cathodic hydroxide coating 17 and the portions 13a and 13b Where the anodic particles were located. However, the resistance of the hydroxide film 17 to passage of electrons is so great as to polarize the cathodic portion and prevent any significant corrosion from occurring.

The foregoing mechanism is also effective in providing substantial protection against atmospheric corrosion and tarnishing. In the case of immersion corrosion, it appears that relatively low potential diiferences between matrix and anode from 50-250 millivolts are desirable. How- 3 ever, in the case of protection against atmospheric corrosion, higher potential differences of the order of 400- 600 millivolts appear more advantageous.

So far, this application has referred to the use of elemental matrices such as copper and iron. It is, of course, not necessary that the matrix he of pure metals only, since hydrous oxide formation produces films that will not protect against concentration cell corrosion. Accordingly, it is most desirable to provide a matrix of substantial resistance to concentration cell corrosion such as would be provided by the addition of strong film formers and to provide improved protection against the obvious disadvantages of film formers such as pitting corrosion by the mechanism described above. These film formers include, for example, Al: 0.5l%, Si: 0.2%, Cr: 0.5 20%, Mn: 02-10%, Ni: 0.5-40%, Zn: -35%, Be: ODS-4%, Mg: 0.012%, Ti: 0.0l5%, Zr: 0.012%, and Co: .0l-4% by weight or combinations of these elements.

It should finally be noted that there is no reason for the anode material to be an element. Alloys can be used for the anode material if they are specifically desirable.

The method of making such two phase metallic mixtures is not critical. Where the two phases show little or no tendency to interact, such as in the case of copper and iron, the mixtures may be made by conventional wrought metallurgy techniques.

In the case of mixture pairs that interact at elevated temperature such as, for instance, copper and aluminum, or iron and aluminum, a powder metallurgy method is clearly necessary to make such products. The particular powder metallurgy method selected is not critical provided that complete interaction between the metallic con stituents is avoided and provided that the resulting mixtures possess structural integrity.

The following observations exemplify but do not limit the scope of the present invention.

SELECTION OF ELEMENT PAIRS (A) Matrix mixtures The corrosion potential of copper in 0.5 M sodium chloride and other nearly neutral electrolytes is around +0.050 volt on the standard hydrogen scale. Any elemental anode that has a potential of +0040 volt, or less is suitable for consideration subject to the previously mentioned auxiliary requirements.

A typical list of elemental additions and their corrosion potentials in 0.5 M NaCl is given below in Table I.

TABLE I corrosion potential on the Element: hydrogen scale-volts Zinc .81

Indium .24

Cadmium .55

Cobalt -.13

Nickel .08

Aluminum .55

Iron .44 Magnesium l.6

From a galvanic standpoint, clearly, all of these metals are anodic to copper in V2 M sodium chloride solution at 25 C.

If it is desired to make'iron matrix composites, clearly, elements with corrosion potentials more active then .45 volt must be selected.

In the case of zinc matrix alloys elements having potentials more active then .82 volt must be selected.

(B) Precipitation of hydrous oxides from the elemental anodes In theory, it ought to be possible to determine whether a hydrous oxide of the above elements will precipitate from an aqueous solution at any given pH merely by reference to standard solubility product data. Unfortunately, a review of solubility product data shows that solubility product data reported for the same metal hydrous oxides will exhibit variations of as much as a factor of 10 Clearly, such information is of little use in predicting whether precipitation will or will not occur.

Accordingly, solutions containing 20 milligrams of the following elements were made up in acid solution: nickel, cadmium, zinc, titanium, indium, niobium, vanadium, molybdenum, thallium, iron, aluminum, cobalt and magnesium. The solutions were then neutralized to pH 7 either with caustic soda or ammonia and the solutions examined for evidence of visible precipitation.

All of the above elements with the exception of niobium and thallium gave visible precipitation at pH 7, indicating hydrolysis to a hydrous oxide, notwithstanding the presence of a low concentration of the metallic cation in the original solution.

Such elements (excepting niobium and thallium) are therefore suitable for inclusion as elemental anodes, provided that they fulfill the corrosion potential requirements outlined above and provided that they fulfill the electrophoretic requirements described in the following section.

(C) Electrophoretic requirement The build-up of hydrous oxide in solution will not contribute to corrosion protection unless the hydrous oxide will migrate electrophoretically and deposit on the matrix cathode. In order to throw light on this point, suspensions of various hydrous oxides were prepared by neutralization of acid solutions; two electrodes of copper were inserted into the solution at 25 C. and an applied current of 28 microamps passed between the electrodes. The cathodes were then weighed to determine the accumulations, if any, of electrophoretically deposited hydrous oxides.

The results showed that all hydrous oxides described in Section I(B) were capable of being deposited on a copper cathode. The particular weight gains are described in Table II.

TABLE II Weight deposited on a copper cathode Metal Ion: mg./cm. /a. in hours Zn+ 17 (D) Corrosion protection by the hydrous oxides It is still necessary to show that the hydrous oxides once ofrmed by galvanic corrosion of the elemental anode and deposited on the matrix cathode can affect the overall corrosion process. This can be relatively simply done by depositing the hydrous oxide electrophoretically on a metal (copper) cathode and subsequently determining the slope of the cathodic polarization curve. This experiment is best done in chromic acid since it prevents reductive dissolution of hydrous oxides such as 'yFe0.0H which have two valency states.

The shape of the cathodic polarization curve on copper is essentially flat; at currents in excess of 1 microamp per sq. cm., polarization of less than 1 microvolt per microamp is observed for pure copper. By contrast, much higher slopes of the order of 200 millivolts per microamp were obtained for the cathodes carrying the hydrous oxides of zinc, iron, aluminum, nickel, cobalt and cadmium. The high slopes of the cathodic polarization curves imply ohmic interefrence with the half cell cathodic corrosion reaction, i.e., the reduction of oxygen dissolved in the electrolyte or the evolution of gaseous hydrogen.

SPECIFIC EXAMPLES The following examples illustrate the invention without limiting its scope.

EXAMPLE I Two mixtures comprising essentially pure iron and es-' sentially pure copper, each mixed with .9% by weight of elemental aluminum were prepared.

The mixtures were prepared from 300 mesh powders of pure iron, copper and aluminum. The copper and aluminum, and iron and aluminum powders were blended completely and compacted into a green compact at a pressure of 150 tons/sq. in. The green compacts were heated to 650 F. in the case of copper and 800 F. in the case of iron and rolled 65% in one pass into the form of strip. The preheating temperatures were insuflicient to cause detectable reaction between the metallic components of the mixtures. The resulting strip had a densification of in excess of 95% and was structurally sound.

The two mixtures were then subjected to corrosion tests for a period of 32 days in .5 M sodium chloride solution. The sodium chloride solution was held at 25 C. with the exception of 1 hour in each day when it was heated and maintained at the boiling point. Control specimens of pure copper and copper-aluminum were also tested as controls.

The results of the corrosion test are shown below in Table III.

TABLE III Weight of Cu or Fe Weight of Al dissolved, dissolved, Metal or mixture mgJcm. rug/em.

Copper 5. 5 (In-.9% A1 056 4. 7 Fe 36. 2 Fe.9% Al 0. 87 ll. 34

EXAMPLE II In order to test whether a protective film of hydrous alumina had formed on both samples of Example I, they were subjected to cathodic polarization tests in /2 M sodium chloride solution.

In the case of copper-aluminum mixture, this curve had a slope of 300 millivolts per microamp compared with less than 1 millivolt per microamp in the case of essentially pure copper.

In the case of iron-aluminum mixture, the slope of the cathodic polarization curve Was approximately 75 millivolts per microamp, as opposed to approximately 35 millivolts per microamp for unfilmed iron.

These results show that not only did the aluminum particles give substantial protection against corrosion of the iron and the copper, but also that this protection was effected by the deposition of hydrous aluminum oxide films on the copper and iron matrices by the electrophoretic mechanism described previously.

6 EXAMPLE III 0.5% by weight of aluminum -.l5% tin alloy powder having a potential of about 1.2 volts in 0.5 M NaCl in a matrix of aluminum 7% magnesium alloy sheet was cold rolled 60% and stabilized at 300 F. for 4 hours.

This material resulted in high stress corrosion resistance for prolonged periods in excess of 100 days when tested at 80% of the yield strength in /2 M sodium chloride solution.

The same Al 7% mg. alloys, free from aluminumtin alloy powder, failed by stress corrosion cracking under the same conditions within one day.

It is to be understood that the invention is not limited to the illustrations described and shown herein which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modifications of form, size, arrangement of parts and detail of operation, but rather is intended to encompass all such modifications which are within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

l. A corrosion resistant article having at least one surface in aqueous solution, said article having a cathodic metallic matrix which has structural integrity interspersed with an anodic material, said anodic material having a corrosion potential in said solution of at least 10 millivolts more active than the matrix material, said anodic material being present within the range of 0.005% to 25% by weight, an insoluble hydroxide coating of said anodic material formed by exposure to said solution, said coating being deposited on at least a portion of said surface and having a high resistance to the flow of electrons therethrough.

2. An article according to claim 1 wherein said anodic material is present within the range of 0.1% to 5% by weight.

3. An article according to claim 1 in which the anodic material has a corrosion potential of at least 50 milli volts more active than said matrix.

4. An article according to claim 1 in which at least a portion of said surface not having a hydroxide coating thereon contains anodic material in contact with said solution.

5. An article according to claim 1 in which the matrix is selected from the group consisting of metallic iron and iron alloys.

6. An article according to claim 1 in which the matrix is selected from the group consisting of copper and copper alloys.

7. An article according to claim 1 in which the matrix is selected from the group consisting of aluminum and aluminum alloys.

8. An article according to claim 1 in which the matrix also contains at least one film former.

9. An article according to claim 1 in which the film formers are selected from the group consisting of Al, Si, Cr, Mn, Ni, Zn, Be, Mg, Ti, Zr, and C0.

References ited UNITED STATES PATENTS 2,286,240 6/1942 Stack 204l23 2,286,241 6/1942 Stack 204l23 3,206,385 9/1965 Meiklejohn et a1 204l23 3,388,987 6/1968 Bailey 204-197 3,448,034 6/ 1969 Craft et a1. 204293 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R. 

